Electrical current responsive instrument with magnetic flux adjustment

ABSTRACT

A current responsive mechanism for electrical instruments of the type having an armature mounted conductor means positioned within the air gap between a pair of spaced magnetic poles of opposite polarity, whereby current flow through the conductor means produces a driving force on the armature in one direction of its movement. The mechanism is provided with a magnetically permeable pole plate which seats against and is releasably secured to one magnet face in such a way that the plate may be adjusted in its edgewise direction to alter the magnetic flux distribution in the air gap for calibration purposes. The disclosed embodiment of the invention is a long scale vertical gap electrical indicating instrument, or meter, wherein the pole plate is adjustable to provide the meter with a linear current response throughout the range of armature rotation.

United States Patent Inventors Henry Marusek Tujunga; Edwin L. Schwartz, Los Angeles, Calif. Appl. No. 789,158 Filed Jan. 6, 1969 Patented Mar. 9, 1971 Assignee Rite Autotronics Corporation Los Angeles, Calif.

ELECTRICAL CURRENT RESPONSIV E INSTRUMENT WITH MAGNETIC FLUX ADJUSTMENT 4 Claims, 7 Drawing Figs.

US. Cl 324/150 Int. Cl G0lr l/20, GOlr 5/08 Field of Search 324/150,

Primary Examiner-Alfred E. Smith Attorney Robert E. Geauque ABSTRACT: A current responsive mechanism for electrical instruments of the type having an armature mounted conductor means positioned within the air gap between a pair of spaced magnetic poles of opposite polarity, whereby current flow through the conductor means produces a driving force on the armature in one direction of its movement. The mechanism is provided with a magnetically permeable pole plate which seats against and is releasably secured to one magnet face in such a way that the plate may be adjusted in its edgewise direction to alter the magnetic flux distribution in the air gap for calibration purposes. The disclosed embodiment of the invention is a long scale vertical gap electrical indicating instrument, or meter, wherein the pole plate is adjustable to provide the meter with a linear current response throughout the range of armature rotation.

PATENTEUHAR 9m 3569832 sum 1 BF 2 HENRY MA/PuSE/r [an/W A. 604MHz INVENTOR.

ATTOR/VA'Y PATENTEDQAR 9 1971 SHEET 20F 2 I'NVEN'IOR. 7

"BY I ELECTRICAL CUENT RESPONSIVE INSTRUMENT WITH MAGNETIC FLUX ADJUSTMENT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to electrical instruments. More particularly, the invention relates to electrical instruments of the class having an electrical current responsive mechanism embodying a movable armature mounting an electrical conductor means which is positioned within the magnetic field between a pair of spaced magnetic poles of opposite polarity in such a way that current flow through the conductor means produces a driving force on the armature in one direction of its movement.

2. Prior Art Electrical instruments of the class described are well known in the art and are utilized for a variety of purposes, such as exercising a control function, providing a readout related to current flow through the conductor means, and other applications. In most if not all instruments of this type, the magnetic flux within the air gap containing the armature conductor means has an ideal or optimum distribution for each particular application. Generally stated, a primary aspect of the present invention is concerned with attaining this optimum flux distribution. 1

As will appear from the ensuing description, the present inventive concept may be applied to .many if not all electrical instruments of the class described. However, the invention is primarily concernedwith and will be disclosed in relation to electrical indicating instruments of this type which are commonly referred to as long scale vertical gap instruments.

Long scale, vertical gap electrical indicating instruments, or meters, are well known in the art and are utilized for a variety of purposes. Instruments of this type, for example, are used as current indicating instruments, electrical tachometers, and the like. Generally speaking, a long scale vertical gap instrument is characterized by an electrical current responsive mechanism including a generally annular magnet having opposite north and south pole faces, a generally annular core concentrically disposed opposite and in spaced parallel relation to one pole face of the magnet, and a magnetically permeable yoke which inductively couples and provides a magnetic flux path between the core and the opposite pole face of the magnet. Mounted for rotation about the common axis of the magnet and core is an armature. This armature carries a coil which encircles the core in a plane containing the rotation axis of the armature. One leg of the coil extends through the air gap between the core and the confronting pole face of the magnet. The instrument is provided with terminals which are electrically connected internally of the instrument to the ends of the coil conductor and are accessible externally for connection to an external electrical current source.

When the armature coil is energized from the external current source, the magnetic field produced by the resulting current flow through the coil reacts with the magnetic field within the air gap between the magnet and core to produce a torque on the armature in one direction about its rotation axis. Rotation of the armature in this direction is resisted by a spring in such a way that the armature assumes a stationary position of equilibrium related to the current flow through the armature coil. Attached to the armature is a needle which rotates with the armature along a scale that is calibrated to provide a readout related to the magnitude of the current flow through the coil.

A primary advantage of such indicating instruments resides in the fact that they have a substantial operating range; that is to say, the armature is designed to rotate through a relatively large angle, typically on the order of 250. As a consequence, the instrument may be arranged to either respond to a relatively large range of current flows or to respond to a reduced range of current flow with high read-out accuracy.

The electrical current responsive mechanism, described thus far, exhibits one deficiency which the currently available long scale vertical gap instruments seek to avoid. The deficiency referred to resides in the fact that the material from which permanent magnets are made are relatively nonhomogeneous and contain blow holes and the like. As a consequence, the magnetic field emanating from the magnet is not uniform over the entire magnet face area. This tends to produce in the air gap of a large scale vertical gap indicating instrument a nonuniform magnetic flux distribution which, if not corrected, would cause the current response of the instrurnent to vary widely over the range of armature movement. In this case, an accurate read-out could be attained only by individual calibration of the scale of each instrument. This, obviously, would be a difficult and time-consuming procedure and would result in high instrument cost.

The above problem is alleviated to some degree in the existing large scale vertical gap instruments by the use of a highly homogeneous, magnetically permeable pole plate. This pole plate has a generally annular shape conforming approximately to that of the magnet and seats against the pole face of the magnet adjacent the air gap. The plate functions, in effect, as a magnetic flux diffuser which diffuses or distributes the magnetic flux from the magnet in such a way as to provide generally uniform flux intensity across the entire air gap surface area of the pole plate.

Adjustment of the pole plate relative to the magnet alters the magnetic flux distribution within the air gap. During manufacture, the pole plate is adjusted to a position wherein the magnetic flux distribution in the air gap approaches, as closely as possible, the ideal or optimum flux distribution. This optimum flux distribution is one in which the magnetic flux intensity is uniform along the entire path of rotation of the armature coil. This optimum or uniform flux distribution provides the instrument with a linear current response over the entire range of armature rotation and thus permits the use of a linear instrument scale which may be conveniently mass produced.

The existing instruments possess one deficiency which the present invention seeks to overcome. This deficiency resides in the fact thatafter adjustment, the pole plate is brazed, bonded, or otherwise permanently joined to the permanent magnet. This method of attaching the pole plate to the magnet has several disadvantages. A primary disadvantage is that it prevents recalibration of the instrument during use. This obviously is a severe disadvantage since the initial calibration of the instrument may be easily disturbed during use as a result of dropping of the instrument or other occurrences. Moreover, permanent attachment of the pole plate to the magnet by brazing, bonding, or the like, is difficult to accomplish, at least without disturbing the optimum setting of the pole plate, and interferes with mass production of the instrument. As a consequence, most existing instruments of this type are quite costly.

SUMMARY OF THE INVENTION The present invention provides an improved electrical instrument of the class described which is characterized by releasable attachment of the pole plate to the magnet. As a consequence, the plate may be readily adjusted relative to the magnet to attain optimum magnetic flux distribution within the instrument air gap and then locked in position to preserve, with a high degree of accuracy, such optimum flux distribution. Moreover, the pole plate can be released and readjusted at any time for the purpose of recalibrating the instrument, when necessary. The invention also adapts the instrument to manufacture by mass production techniques and thereby lowers the instrument cost.

As noted earlier, the basic improvements of the invention may be utilized to advantage in a variety of electrical instruments of the character described. The particular embodiment of the invention which has been selected for presentation in this disclosure is a long scale vertical gap indicating instrument, or meter, of the type discussed above. The armature of this meter rotates in jewel bearings carried by adjustable supports, one of which extends centrally through the magnet and pole plate of the instrument. These bearing supports must be adjusted with great precision to assure proper journaling of the armature in the bearing jewels. A feature of the disclosed embodiment resides in the fact that the pole plate may be adjusted without disturbing this precise setting of the jewel bearing supports. To this end, the pole plate is centrally apertured to receive the adjacent jewel bearing support with a relatively loose fit, such that the plate may be adjusted edgewise relative to the magnet without interference from the bearing support. The plate is releasably secured to the magnet by a tubular nut threaded on the bearing support. This nut has a flange at one end which engages over the inner edge of the plate to firmly clamp the latter in fixed position.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a front view of an electrical instrument embodying an improved current responsive mechanism according to the invention;

FIG. 2 is a side elevation of the instrument in FIG. 1;

FIG. 3 is a section taken on line 3-3 in FIG. 2;

FIG. 4 is an enlarged section taken on line 4-4 in FIG. 1;

FIG. 5 is a section taken on line 5-5 in FIG. 4;

FIG. 6 is a section taken on line 6-6 in FIG. 4 with the pole plate in one position of adjustment and FIG. 7 is a view similar to FIG. 6 with the pole plate in another position of adjustment.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to these drawings, there is illustrated an electrical instrument l0 embodying a current responsive mechanism 12 according to the invention. Described in general terms, this mechanism includes magnetic circuit means 14 having spaced confronting pole faces 16 and 18 of opposite polarity. Disposed in seating. contact with one pole face, in this case pole face 18, is a pole plate 20 of magnetically permeable material. The pole plate defines with the other pole face 16 an intervening air gap 22. Pole plate 20 is adjustable in its edgewise direction relative to its seating pole face 18 to alter the magnetic flux distribution within the air gap 22. Releasable fastening means 24 are provided for steering the pole plate in fixed position relative to the pole face 18. It will be understood, therefore, that the fastening means may be released to adjust the pole plate in its edgewise direction relative to the pole face 18.

Also embodied in the current responsive mechanism 14 of the instrument is an armature 26. This armature includes electrical conductor means 28 disposed within the air gap 22. Armature 26 is movably mounted by bearing means 30 for movement of its conductor means 28 along the air gap 22. The instrument is equipped with terminals 32 which are electrically connected internally of the instrument to the armature .conductor means 28. These terminals are accessible externally of the instrument for connection to an external electrical current source.

When the instrument terminals 32 are electrically connected to an external current source, a current flow occurs through the armature conductor means 28. This current flow creates about the conductor means a magnetic field which interacts with the magnetic field within the instrument air gap 22 to produce on the armature 26 a driving force in one direction of its movement.

It will now be understood, therefore, that when the armature conductor means 28 is energized from the external current source, a magnetic driving force is exerted on the armature 26 in a direction to move the latter against the action of its spring 34. As a consequence, the armature assumes a stationary position of equilibrium wherein the driving force and the spring force are balanced. It will be evident to those versed in the art that the driving force exerted on the armature, and

hence the stationary position of equilibrium assumed by the armature, are determined by two parameters. These parameters are the current flow through the conductor means 28 and magnetic flux distribution within the instrument air gap 22. Edgewise adjustment of the pole plate 20 alters this magnetic flux distribution. A primary feature of the invention resides in the releasable fastening means 24, whereby the pole plate 20 may be readily released and adjusted to alter the air gap flux distribution. During initial manufacture of the instrument, the pole plate is adjusted to attain the ideal or optimum flux distribution and to initially calibrate the instrument, after which the plate is locked in its optimum setting. At any time during subsequent use of the instrument, the plate may be released and readjusted to reestablish the optimum flux distribution of the air gap and to recalibrate the instrument. The particular instrument illustrated is a long scale vertical gap indicating instrument of the type referred to earlier. In this type of instrument, the ideal or optimum air gap magnetic flux distribution is one in which the magnetic fiux intensity is uniform throughout the'total range of movement of the instrument armature 26, and more specifically throughout the range of movement of the armature conductor means 28 in the air gap 22. As will appear from the ensuing description, the present invention permits adjustment of the pole plate to reestablish this uniform flux distribution at any time. The advantage of the uniform air gap flux distribution in a long scale vertical air gap instrument resides in the fact that it provides the instrument with a linear current response throughout the range of movement of the armature 26.

Referring now in greater detail to the drawings, the magnetic circuit means 14 of the illustrated long scale vertical gap instrument 10 comprises a generally annular permanent magnet 36 having a sector shaped cutout 37. The pole face 18 is located at the front side of this magnet. The magnet has a rear pole face 38 of opposite magnetic polarity to the front pole face 18. Seating against the rear pole face 38 is a magnetically permeable yoke plate 40. This yoke plate extends across the rear side of the magnet cutout 37. Extending through this cutout and seating at its rear end against the yoke plate 40, is a generally gore-shaped post 42. The front end of this post projects a distance forwardly of the front pole face 18 of the magnet 36. A generally annular core 44 of magnetically permeable material seats against the front protruding end of the post 42. The rear face of this core provides the pole face I6 and is spaced from the adjustable pole plate 20 to define the air gap 22. Projecting from the front end of the post 42 into a complementary shaped cutout in the core 44 is a tongue 46. This tongue locates the core concentrically with the magnet 36. As shown best in FIG. 7, the walls of the cutout 37 in the magnet 36 are spaced from the sidewalls of the post 42. It will be evident, therefore, that the yoke plate 40 and post 42 magnetically couple and provide a magnetic flux path between the rear pole face 38 of the magnet 36 and the magnetically permeable core 44. It will be further evident that the rear face '16 of the core 44 and the front face 18 of the magnet 36 comprise magnetic poles of opposite polarity.

Forwardly of the core 44 is an instrument frame member 48 of nonmagnetically permeable material. This frame member has a generally annular configuration and is concentrically disposed relative to the core 44 and the magnet 36. The upper portion of the frame member 48 in FIG. 4 provides a rearwardly projection post 50 which seats against the front side of the core 44. Locating pins 52 (FIG. 5) project rearwardly from this post into mating bores in the core 44. These locating pins retain the frame member in fixed coaxial relation to the core. Extending through the rear yoke plate 40 and the post 42 into the post 50 of the from frame member 48, and threaded in the latter post, is a bolt 52. This bolt joins the magnet 36, yoke plate 40, post 42, core 44, and front frame member 48 in rigid assembled relation.

The instrument armature 26 has a central shaft 54 on the common axis a of the magnet 36 and core 44. Fixed to and extending laterally of this shaft, forwardly of the core, is an arm 56. The armature conductor means 28 is a coil which surrounds the core 44 in a plane containing the armature axis a and is fixed to one end of the arm 56. The coil thus rotates with the armature about its rotation axis and circumferentially along the core 44. During this rotation, the inner side or leg of the coil, which is positioned within the air gap 22, moves through the air gap in the plane of the gap. The instrument terminals 32 are connected to the ends of the wire conductor of the coil 28 through the armature springs 34. Fixed to the other end of the arm 56 is a pointer 58 which extends forwardly through a central opening in a scale plate 60 and then laterally across the front side of the plate. Scale plate 60 is secured to the front frame member 48 and bears a scale 62. The pointer 58 moves along this scale during rotation of the armature 26 in response to current flow through the armature coil 28. The scale is suitably calibrated to provide the desired read-out related to the magnitude of current flow.

The armature bearing means 30 comprise jewel bearings fixed within the ends of a pair of adjustable bearing screws 64 and 66. Bearing screw 64 is threaded in a tubular bearing support 68. This bearing support is staked at its rear end to the yoke plate 40 and extends centrally through the magnet 36 on its axis a. The bearing screw 66 is threaded in a bearing support 70 which extends through and is fixed to the front frame member 48 on the axis a. During manufacture and initial calibration of the instrument, the bearing screws 64, 66 are adjusted to properly locate the armature 26 in its axial direction as well as to properly journal the armature in the bearing jewels. This adjustment requires setting of the bearing screws with great precision. Accordingly, it is undesirable to later disturb the bearing screw setting. 7

At this point, attention is directed to the fact that the instrument structure described thus far, with the exception of the adjustable pole plate and its releasable fastening means 24, is conventional. However, such instrument structure has been described with some degree of detail in order to provide a better background for and understanding of the present improvements which reside in the pole plate and its releasable fastening means. i

As noted earlier, the instrument illustrated is a long scale vertical gap instrument. In an instrument of this type, the armature 26 is designed to rotate through a relatively large angle in response to current flow through its coil 28. For example, the angle of armature rotation is typically on the order of 250. It is desirable in such instruments to have a uniform magnetic flux distribution, that is, a uniform magnetic flux intensity, within the air gap 22 throughout the range of armature rotation. This uniform flux distribution provides the instrument with a linear response to current flow through its armature coil 28 throughout the range of armature rotation and permits the use of a linear read-out scale 62 which may be easily mass produced.

it is well known in the art that the magnet 36, without the pole plate 20, is incapable of providing such uniform flux distribution. This is due to the nonhomogeneous nature of and the existence of blow holes and other aberrations in the magnetic material of permanent magnets. As noted earlier, the pole plate is constructed of a highly homogeneous magnetically permeable material which acts to diffuse the nonuniform magnetic field of the magnet into a generally uniform magnetic field within the air gap 22. During manufacture of the instrument, the latter is calibrated, in part, by adjusting the pole plate edgewise to attain the ideal or optimum flux distribution in the air gap. The plate is then fixed in its optimum setting. l-leretofore, this has been accomplished by brazing, bonding, or otherwise permanently attaching the plate to the adjacent pole face l8 of the magnet 36. This method of attachment of the plate is undesirable for the reasons stated earlier.

According to the present invention, the pole plate 26 is releasably secured to the magnet by the fastening means 24. This fastening means is uniquely constructed and arranged to facilitate mass production and initial calibration of the instrument as well as subsequent recalibration of the instrument when necessary. To these ends, the fastening means comprises a clamp nut or collar 72 surrounding and threaded on the front end of the rear armature bearing support 68. This collar is situated within the central opening 74 in the pole plate 20 and is formed with a circumferential flange 76 which projects laterally over the front side of an axially reduced shoulder 78 surrounding the opening. As shown best in H6. 6, the pole plate opening 74 is considerably larger in diameter than the tubular body of the clamp collar 72. As a consequence, when the collar is backed off from clamping engagement with the pole plate 20, the latter may be freely adjusted in all edgewise directions relative to the magnet 36. The plate may be then secured in fixed position by threading the clamp collar toward the magnet 36 into clamping engagement with the pole plate. It will be understood that the collar is provided with suitable wrench-engaging means for turning the collar to release and tighten the collar.

It is now evident that the pole plate 20 may be easily ad- 20 justed in its edgewise direction to initially calibrate the instrument during manufacture and to later recalibrate the instru-. ment, when necessary. After adjustment, the plate may be securely locked in fixed position until subsequent recalibration is again necessary. A significant'advantage of the invention, in addition to this recalibration capability, resides in the fact that the pole plate may be adjusted without disturbing the precise setting of the armature bearing means 30. It will also be obvious to those versed in the art that the present instrument is ideally suited to manufacture by mass production techniques.

Accordingly, while the invention has herein been shown and described in what is conceived to be its most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope of the invention, which is not to be limited to the details disclosed herein, but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices.

We claim:

1. A current responsive mechanism for electrical instruments of the class described comprising:

a magnetic structure including an annular permanent magnet having flat magnetic pole faces of opposite magnetic polarity at opposite sides of said magnet, an annular magnetically permeable core coaxially disposed opposite and in spaced parallel relation to one pole face of said magnet, and magnetic circuit means providing a magnetic flux path between-said core and the other pole face of said magnet; 1

an annular magnetically permeable pole plate disposed in seating contact with said one pole face of said magnet and defining with said core an intervening annular air gap, there being a central opening through said magnet, core and pole plate;

said pole plate being adjustable edgewise relative to said one pole face to establish a generally uniform magnetic flux distribution throughout the full circumferential length of said air gap;

releasable means securing said pole plate in fixed position relative to said one pole face, whereby said securing means may be released to adjust said pole plate;

an armature including a shaft extending through said opening on the common axis of said magnet and core, a coil fixed on said shaft and surrounding said coil, and coil terminals for connection to an external electrical current source; bearing means on said structure rotatably supporting said v shaft for rotation of said armature on said axis;

the central opening in said pole plate being sized to permit edgewise adjustment of said plate without disturbing said bearing means;

said securing means comprises a threaded clamp collar on said structure surrounding said armature shaft and having a clamping flange extending laterally over the inner edge of said plate; and

said collar is rotatably into and from clamping engagement with said pole plate. 2. A current responsive mechanism according to claim 1 wherein:

said armature bearing means comprise a pair of bearing sup- 5 ports on said axis at opposite sides of said air gap, whereby one bearing support is located at the same side of said air gap as said magnet and pole plate;

said armature shaft extends between said bearing supports and has its ends journaled in said bearing supports;

said clamp collar includes a tubular shank extending through the central pole plate opening and threaded on said one bearing support, whereby said collar may be adjusted to release and clamp said pole plate, and the central pole plate opening is substantially larger in diameter than said collar shank, whereby said pole plate may be adjusted without disturbing said bearing means.

3. A current responsive mechanism according to claim 2 wherein:

said instrument is a long scale vertical gap indicating instrument having a scale plate in a plane normal to said axis;

said armature includes a needle positioned in front of and rotatable along said scale plate during rotation of said armature, and

said scale plate is provided with a read-out scale having linear scale divisions. 4. A magnetic assembly for electrical instruments of the class described comprising:

an annular permanent magnet having flat north and south pole faces at opposite sides of said magnet;

an annular magnetically permeable pole plate disposed in seating contact with one pole face;

said pole plate being adjustable edgewise relative to said magnet;

releasable fastening means securing said pole plate to said magnet, whereby said fastening means may be released to adjust said plate edgewise relative to said magnet;

said releasable fastening means comprises a bearing post fixed to said magnet and extending centrally through the opening in said magnet and pole plate, a clamping collar having a tubular shank threaded on said bearing post and a clamping flange projecting laterally from said shank over the inner edge of said pole plate at the side of said plate remote from said magnet, whereby said collar is rotatable in one direction to clamp said pole plate against said magnet and in the opposite direction to release said pole plate for edgewise adjustment relative to said magnet; and

the diameter of the central opening in said pole plate is substantially greater than the diameter of said collar shank, whereby said plate may be adjusted edgewise relative to said magnet. 

1. A current responsive mechanism for electrical instruments of the class described comprising: a magnetic structure including an annular permanent magnet having flat magnetic pole faces of opposite magnetic polarity at opposite sides of said magnet, an annular magnetically permeable core coaxially disposed opposite and in spaced parallel relation to one pole face of said magnet, and magnetic circuit means providing a magnetic flux path between said core and the other pole face of said magnet; an annular magnetically permeable pole plate disposed in seating contact with said one pole face of said magnet and defining with said core an intervening annular air gap, there being a central opening through said magnet, core and pole plate; said pole plate being adjustable edgewise relative to said one pole face to establish a generally uniform magnetic flux distribution throughout the full circumferential length of said air gap; releasable means securing said pole plate in fixed position relative to said one pole face, whereby said securing means may be released to adjust said pole plate; an armature including a shaft extending through said opening on the common axis of said magnet and core, a coil fixed on said shaft and surrounding said coil, and coil terminals for connection to an external electrical current source; bearing means on said structure rotatably supporting said shaft for rotation of said armature on said axis; the central opening in said pole plate being sized to permit edgewise adjustment of said plate without disturbing said bearing means; said securing means comprises a threaded clamp collar on said structure surrounding said armature shaft and having a clamping flange extending laterally over the inner edge of said plate; and said collar is rotatably into and from clamping engagement with said pole plate.
 2. A current responsive mechanism according to claim 1 wherein: said armature bearing means comprise a pair of bearing supports on said axis at opposite sides of said air gap, whereby one bearing support is located at the same side of said air gap as said magnet and pole plate; said armature shaft extends between said bearing supports and has its ends journaled in said bearing supports; said clamp collar includes a tubular shank extending through the central pole plate opening and threaded on said one bearing support, whereby said collar may be adjusted to release and clamp said pole plate, and the central pole plate opening is substantially larger in diameter than said collar shank, whereby said pole plate may be adjusted without disturbing said bearing means.
 3. A current responsive mechanism according to claim 2 wherein: said instrument is a long scale vertical gap indicating instrument having a scale plate in a plane normal to said axis; said armature includes a needle positioned in front of and rotatable along said scale plate during rotation of said armature, and said scale plate is provided with a read-out scale having linear scale divisions.
 4. A magnetic assembly for electrical instruments of the class described comprising: an annular permanent magnet having flat north and south pole faces at opposite sides of said magnet; an annular magnetically permeable pole plate disposed in seating contact with one pole face; said pole plate being adjustable edgewise relative to said magnet; releasable fastening means securing said pole plate to said magnet, whereby said fastening means may be released to adjust said plate edgewise relative to said magnet; said releasable fastening means comprises a bearing post fixed to said magnet and extending centrally through the opening in said magnet and pole plate, a clamping collar having a tubular shank threaded on said bearing post and a clamping flange projecting laterally from said shank over the inner edge of said pole plate at the side of said plate remote from said magnet, whereby said collar is rotatable in one direction to clamp said pole plate against said magnet and in the opposite direction to release said pole plate for edgewise adjustment relative to said magnet; and the diameter of the central opening in said pole plate is substantially greater than the diameter of said collar shank, whereby said plate may be adjusted edgewise relative to said magnet. 